/Engine/Source/Runtime/GeometryCore/Public/DynamicMesh/DynamicMeshOverlay.h
Set the triangle to the given Element index tuple, and increment element reference counts
[EMeshResult](API\Runtime\GeometryCore\UE__Geometry__EMeshResult) SetTriangle ( int TriangleID, const [FIndex3i](API\Runtime\GeometryCore\FIndex3i) & TriElements, bool bAllowElementFreeing )